The art has devoted considerable attention to the problem of transmitting data in a high rate between users being at different locations. Such users, may be home Personal Computers (PCs), office desktop workstations, cable television broadcasting services, Local Area Networks (LANs) and others. In some applications users are connected to each other by modems (modulator-demodulator) which encode the digital data to be delivered from one point (user) to another point and transmit the encoded data through a data link which may be, for instance, an analog communication channel. Such data comprises voice, digital video movies and software data files.
Digital Subscriber Loops (DSLs) comprise several technologies for high data rates, e.g., Asymmetric Digital Subscriber Loops (ADSLs), High speed Digital Subscriber Loops (HDSLs) and Very High speed digital Subscriber Loops (VDSLs). Generally, the whole family of DSLs is commonly known as χDSL. In some VDSL applications, like video transmission, data should be transmitted in very fast rates, usually up to 12.96 Mb/Sec or even exceed 25.92 Mb/Sec.
Analog modems were developed to deal with data rates up to 33.6 Kb/Sec. This rate is unacceptable for many applications, e.g., picture transmission where pictures are constructed from large data files. Digital modems which are developed to work on leased copper lines between two locations can reach higher data rates, up to 64 Kb/s or even 128 Kb/s. However, this rate is still too low for many applications. Any transmission medium interferes with the transmitted data by adding noise, by attenuating its amplitude, and by changing its phase. Digital modems suffer from these phenomena, reducing their ability to receive data without errors. Errors are critical in digital modems.
LANs are very intensively used to connect users, usually in the range of a single building but in many cases the range is expanded to several buildings. Since in many cases it is desired to connect users being in different buildings to share same data base, it is generally desired to exploit for this purpose an existing PSTN twisted pair line, or preferably a leased line. Moreover, in many cases it is desired to make high rate data communication between two LANs, for example, LANs of two offices located in different cities a hundred miles or more away from one another.
There are known connections that can provide higher bandwidth than twisted pair copper lines, for example, 10/100-Base-T coaxial cables and fiber-optic lines. The copper lines between PABXs were already replaced by fiber-optic lines in most cases, and have become standard. However, it is not foreseen that in the near future the twisted pair copper lines between the telephone end users and the PABXs be replaced, due to their hugh number, and to the complexity of replacing them. Therefore, it is desirable to provide a much higher rate modem communication on the relatively narrow bandwidth twisted pair copper lines. Significant efforts are now put in order to develop higher rate modems, which are commonly called in the art, VDSL modems.
Basically, the conventional unshielded copper wire twisted pair was originally designed to provide a medium for voice transmission, and when it is used in telephone communication its bandwidth is confined by filters in its two ends to between 300 Hz to 3.4 KHz. In leased lines, a wider bandwidth is available, however the possible data rate is still limited by the fact that long lines introduce very large attenuation, especially in the higher range of the bandwidth, which exceeds 8 MHz in VDSL modem transmission. This relatively wide bandwidth is required to enable full duplex communication channel, utilizing the known Frequency Division Duplex (FDD). Moreover, telephone lines pass through switching exchanges conducted by the local telephone companies, and this may be a very noisy environment which disrupts the transmitted data.
Usually, digital χDSL modems utilize Quadrature Amplitude Modulation (QAM) techniques to encode data. In this technique, the transmitted information-carrying signal appears in pre-defined amplitude and phase states, each state representing a pre-determined number of bits, and is termed “a symbol”. Conventional QAM techniques utilize 16 states (symbols) or 64 states. In case of 64-QAM, each symbol represents 6 bits. Therefore, for a desirable VDSL modem transmitting at a rate of 12.96 Mb/Sec, 2.16*106 symbols have to be transmitted in each second. A one kilometer twisted pair line has a propagation delay (impulse response time) in the range of about 12 μSec, whereas each symbol duration is 0.463 μSec in the above case. Thus, the effective duration of the line impulse response is about 25 symbols. This long duration of the impulse response of the line leads to a severe Inter symbol Interference (ISI) which may result in a large errors at the receiving modem if cannot canceled, and practically limits the data rate.
The communication between two χDSL modems is carried out while one modem is the transmitter (master) and the other is the receiver (slave). Data directed to the slave modem are termed “downstream” while the data directed to the master modem are termed “upstream”. Communication between the two modems requires synchronization between their timing clocks. Proper operation of χDSL systems requires almost perfect synchronization between master and slave clocks, which means that they must work at the same frequency. Any constant frequency offset leads to a constant growing phase error which may lead to mismatch between the number of transmitted and received symbols per time unit, which is unacceptable. Different clocks always have somewhat different frequencies due to manufacturing tolerances, aging (changes in their component characteristics versus time), temperature variations, power supply tolerances, random noise deviations, etc. Therefore, synchronization means are required in the slave modem to recover the master clock frequency (timing) from the transmitted symbols, together with a correction apparatus to lock the slave clock frequency to the master clock frequency.
One known method for synchronization between receiving and transmitting modem clocks is performed by the transmission of a pilot tone from the master modem to the slave modem. However, in case of pilot tone transmission the energy is concentrated in a single frequency, violating the Power Spectral Density (PSD) constraints and interfering with other systems operating in the same frequency range. It is generally desirable that the power of the synchronizing signal will be distributed on a wide frequency band, but usually these signals are not periodic. Therefore, using distributed power signals for synchronization of χDSL systems is problematic.
Considering the aforementioned problems, an χDSL system is required to synchronize in “blind” mode, which means operating in a very noisy environment when initially there is no information about the transmitted symbols at the receiving modem. This mechanism is known as Blind Timing Recovery (BTR). It is characterized by the fact that all symbols have equal probabilities and some or most of them are attenuated, resulting in a very bad signal to Noise Ratio (SNR) and/or being received with a random phase-shift and with high additive noise. BTR algorithms face significant difficulties when trying to reconstruct the master clock. Thus, an effective error correction mechanism is required, without reducing the data rates.
Several suggested solutions for BTR have been proposed. “Passband Timing Recovery in an All-Digital modem receiver” by D. Godard, IEEE Transactions on Communications. Vol. COM-26. No. 5, 1978, p.p. 517-523, the disclosure of which is incorporated herein by reference describes a method of performing BTR. However, this reference does not provide a mathematical proof, or show any means for carrying it out.
An effort to carry out Godard's method is discussed in “Joint Blind Equalization, Carrier Recovery, and Timing Recovery for High Order QAM Signal Constellation”, IEEE Transactions on Signal Processing, Vol. 40, No. 6, 1992, p.p. 1383-1398 the disclosure of which is also incorporated herein by reference. This reference describes means for performing BTR by applying a complicated algorithm, based on Godard's theory. Particularly, these means require complicated hardware having extremely high processing power.
It is an object of the present invention to provide a synchronization method useful for fast bi-directional data transmission, between χDSL modems over conventional unshielded copper or the like wiring, for example connecting LANs.
It is another object of the present invention to provide a simple fast method for accurately recovering the clock frequency of the transmitting χDSL modem at the receiving modem, without the need of a predetermined training sequence.
It is another object of the invention to provide a method for fast synchronization of the receiving χDSL modem clock to the transmitting χDSL modem clock, while operating in blind mode
It is still another object of the invention to provide adaptive, fast converging error correction apparatus for carrying out the method of the invention.
Other objects and advantages of the invention will become apparent as the description proceeds.